2-hydroxy-4-methylthiobutyric acid (MHA) is the hydroxyl analogue of the essential amino acid methionin in racemic form and like the latter is an important additive in animal nutrition. When rearing poultry, MHA shows similar growth stimulating properties to those of the amino acid recognised for this purpose. The additive is also being used increasingly in other areas of animal nutrition.
MHA is mostly used in the form of aqueous concentrates, wherein these contain, in addition to the monomer, a certain proportion of oligomers, mainly dimeric and trimeric linear ester acids. The actual amount of these oligomers depends on the conditions of preparation and the concentration selected. Due to their low nutritive action and the inconvenient effect on the flow properties, due to an increase in viscosity, it is desirable, however, to keep their percentage proportion as low as possible. Commercial formulations preferably have, for a total concentration of 88-90 wt. %, up to 24 wt. %, corresponding to ca. 27 mol. % in total of oligomers, corresponding to a monomer/oligomer ratio of .about.3:1.
Use of the calcium salt and mixed calcium ammonium salt of MHA as an animal feed additive is also known. Production of these salts, however, is associated with higher production costs. In addition, as powdered solids they are less easy to incorporate into the feedstuff formulation than readily sprayed aqueous concentrates of the free acids with low proportions of oligomers.
The synthetic route to MHA consists of 3 reactions.
The general process for preparing MHA starts from 3-methylthiopropionaldehyde, also called methylmercaptopropionaldehyde or MMP, which is reacted with hydrogen cyanide to give 2-hydroxy-4-methylthiobutyronitrile, also called MMP-cyanohydrin or MMP-CH (equation 1). ##STR1##
The MMP-cyanohydrin produced is then usually hydrolysed with strong inorganic acids via the intermediate stage of 2-hydroxy-4-methylthiobutyramide, also called MHA-amide (equation II), ##STR2## to give the methionin hydroxy analogue (MHA) (equation III). ##STR3##
This hydrolysis can be performed in either one or two stages, wherein "stages" is understood to mean that inorganic acid and/or water is added either once or twice to hydrolyse the MMP-CH, i.e. the number of stages corresponds to the number of addition processes.
A two-stage method of working starting from MMP-cyanohydrin is described in U.S. Pat. Nos. 2,745,745, 2,938,053 and 3,175,000. There, the cyanohydrin is initially reacted at relatively low temperatures with concentrated inorganic acids, e.g. with 50-85% strength sulphuric acid, to give MHA-amide, whereupon hydrolysis to give MHA is then performed at elevated temperature after the addition of water. The calcium or calcium ammonium salt of MHA and the coupled product calcium sulphate is obtained from this by treating the saponification mixture with calcium hydroxide or carbonate. To avoid the inevitable production of unwanted secondary products, the first two patents mentioned recommend using the hydrolysis reagent sulphuric acid in a sub-stoichiometric ratio to MMP-cyanohydrin of, for example, 0.55-0.8:1. British patent 722 024, which describes a similar method of forming the MHA salts from MHA-amide, also implies the two-stage method of working.
A two-stage hydrolysis is also used in the methods disclosed in European patents 0 142 488 (using sulphuric acid) and 0 143 100 (using inorganic acids), the object of which is the recovery of MHA in a liquid form, that is in highly concentrated aqueous solutions. These are obtained after the hydrolysis reaction, performed under defined concentration and temperature conditions via the amide stage using excess inorganic acids, by means of solvent extraction, wherein specific solvents which are partially miscible with water are used.
According to the data in these patents, the characterising feature of the method described therein is regarded as the recovery of MHA from the extraction solution, which is performed in such a way that recovery includes removal of the organic solvent in the presence of at least about 5 wt. % of water, with respect to the remaining extract (MHA). MHA is recovered from the extraction solution by distillation (see the examples), wherein steam distillation is preferred. By removing the solvent from the extraction solution during steam distillation, the bottom product obtained is a mixture of MHA and water. Steam distillation is therefore performed in such a way that the bottom product contains at least 5 wt. % of water.
At another point, the patents in question specify that the column conditions during distillation are controlled in such a way that throughout the column, at least, however, in the bottom fraction, the liquid phase contains about 5 wt. % of water.
From this, it follows that without the presence of a sufficient amount of water during recovery of MHA from the extraction solution the increasing formation of unwanted secondary products (dimers and oligomers) is to be feared.
Furthermore, the steam is used during distillation as a driving agent for complete removal of the extraction agent from the MHA solution, e.g. by forming a low-boiling azeotropic mixture with the corresponding extraction agent.
Concentrated MHA solutions without the use of a solvent are obtained according to U.S. Pat. No. 3,773,927 by means of a two-stage hydrolysis of MMP-cyanohydrin using excess aqueous hydrochloric acid, subsequent concentration of the saponification mixture and separation of the crystallised-out ammonium chloride. The MHA concentrates obtained in this way, however, smell of oligomers and are dark coloured. Also, the isolated ammonium chloride is heavily contaminated.
According to U.S. Pat. No. 4,353,924, the excess inorganic acid, after two-stage hydrolysis using hydrochloric acid, is neutralised with ammonia or another alkaline substance. Concentrated MHA solutions with very few corrosive properties are obtained in this way.
A process is described in U.S. Pat. No. 4,310,690 in which, after hydrolysis with hydrochloric acid the mixture is neutralised with caustic soda solution under precisely defined conditions and the ammonium chloride is converted into common salt and ammonia. During subsequent treatment with slaked lime, the MHA calcium salt is obtained as a slurry in a virtually saturated solution of common salt. After solid/liquid separation, the majority of the filtrate is recycled to prepare the slaked lime slurries. In this way, pollution of the effluent is reduced and the co-production of unwanted or environmentally polluting impurities is avoided.
One-stage hydrolysis methods are also described in the patent literature. Thus, the process according to British patent 915 193 is aimed at the recovery of the MHA calcium salt, wherein after saponification of MMP-cyanohydrin with excess dilute sulphuric acid, the MHA formed is separated from the saponification solution by means of extraction with high-boiling ethers and the MHA calcium salt is obtained by subsequent treatment of the extract with calcium hydroxide. Return of the aqueous raffinate to the saponification stage which is provided in this continuous process, however, leads to an accumulation of inorganic minor constituents.
European patent 0 330 527 discloses a further one-stage hydrolysis process using sulphuric acid as the saponification agent, which takes place without solvents and leads directly to concentrated aqueous MHA solutions, wherein crystalline ammonium sulphate in saleable form is obtained as the co-product. This objective is achieved by neutralising the saponification mixture with ammonium hydroxide solution until the excess inorganic acid and the ammonium bisulphate produced have been converted into the neutral sulphate, wherein two liquid phases are produced, which for their part can be separated and evaporated down in order to obtain MHA on the one hand and crystalline ammonium sulphate on the other hand. In this case, the various filtration and recovery steps are combined in such a way that virtually no product is lost and no salt-polluted effluent is produced. The resulting MHA is of similar quality to the product obtained according to EP 0 142 488.
However, even this environmentally friendly process has various disadvantages. As the applicant of the present invention established when reworking this process, as a result of the comparatively high dilution of the sulphuric acid (20-50%) a considerably higher excess of acid than stated has to be used in order to achieve complete cyanohydrin conversion. Also, to avoid the deposition of salt during neutralisation, high dilutions have to be used in order to be able to separate the two liquid phases cleanly. Next, the ammonium sulphate obtained is of a sticky consistency and associated with a penetrating odour so that aftertreatment, such as e.g. wash-filtration or recrystallisation is required, which increases the costs of the process. Again, the process consumes more energy in the evaporation steps than the process in EP-A 0 142 488 which is cited for comparison, differently than is asserted. Solids handling, arranged in two separate lines with filtering/centrifuging and, not shown in the flow-chart, drying of the ammonium sulphate, is also very costly and utilises complex equipment.